Wireless communication systems have become a major way of communication, replacing conventional land-based communication systems in many applications. There are many types of wireless communication systems, such as, cellular phone systems, wireless local area networks (LAN), WiFi, ad-hoc networks and wireless sensor and control networks used in cyber-physical systems. While wirelessly communicating using a cellular phone is well known, the other wireless communication systems are growing rapidly.
One common problem frequently encountered in wireless interactions between a receiver and a transmitter is the presence of interfering signals or interference originating from devices other than the transmitter and the receiver. Depending on the type of the wireless communication, in some applications, this interference may be intentional, such as the jamming of military wireless transmissions. In other applications, the interference may be accidental, for example, resulting from multiple users sharing a common wireless channel with or without a base station. The presence of such interfering signals can compromise the ability of the receiver to discern the signal from the intended sender, resulting in a reduction, sometimes significant reduction, of information throughput of the wireless transmission from the transmitter to the receiver.
All wireless communication occurs through airwaves divided into various frequency bands. By virtue of using different frequency bands, various types of wireless technologies can use the airwaves simultaneously without interfering with one another. These frequency bands may be either licensed or unlicensed. To transmit over a licensed band, operators typically obtain a license by paying a licensing fee for the exclusive right to transmit on a particular frequency band. Thus, interference on a licensed band may be controlled. On the other hand, communications over an unlicensed band requires no license or special permission and, therefore, interference in the unlicensed spectrum may not be well controlled.
In at least some instances, the interference in an unlicensed spectrum may be significant compared to signal power. This is partly because the signal power may be kept small in an unlicensed spectrum to reduce interference to other user signals. Therefore, the signal to interference ratio (SIR) in an unlicensed spectrum may be very small, for example, around a negative thirty decibels (˜−30 dB). Wireless communication systems, such as WiFi, may use the network layer of the unlicensed spectrum to avoid such interferences. However, communicating in the network layer may limit the throughput and the transmitting distance in the coverage area. More importantly, since communication in the network layer uses a public network, those communications may not be secure. To avoid the disadvantages of transmitting in the network layer, modem wireless communications may occur in the physical layer. However, in the physical layer, the interference may not be well controlled.
Two methods are typically used for mitigating interference in the physical layer. The first method uses a filter such as a low-pass, high-pass and/or a band-pass filter, which takes advantage of the frequency spectrum of the interferences. However, such filters may not be very useful in mitigating co-channel interferences. The second method uses a stochastic filter that takes advantage of the stochastic properties of the interferences. However, such filters may require cross correlations between a measured variable and the interference to be effective. Accurate cross correlation statistics are hard to obtain in the unlicensed spectrum and, without such statistics the stochastic filter may not function properly.
Accordingly, there exists a need for a new filter that may be utilized for effectively countering the negative effects of interfering signals in an unlicensed spectrum for communications in the physical layer.